Television and like transmitting apparatus



May 6, 1952 R. s. WEBLEY TELEVISION AND LIKE TRANSMITTING APPARATUS Filed June 6. 1950 Quantum N awe/11a,-

m. m m D0 M4 m 0 new Patented May 6, 1952 TELEVISION AND LIKE TRANSDHTTING APPARATUS Reginald Sidney Webley, Hayes, England, assignor'to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain 7 Application June 6, 1950, Serial No. 166,369

In Great Britain June 11, 1949 1 This invention relates to television and like transmitting apparatus.

In a television pick-up tube of the kind havinga target of the electrostatic charge storage type which, in operation of the tube, is scanned by an electron beam, considerable advantage can be obtained if, instead of projecting the light image of the object for transmission directly on to the target, said light image is projected on to a photosensitive electrode so as to produce an electron image and the electron image is projected on to the target. Television pick-up tubes of this type, which may be referred to as the electron image type, are described, for example, U. S. Patent specification No. 2,244,466 and a form of tube similar to that illustrated in Figure 1 of said specification has found considerable practical application, tubes of this form operating with anode potential stabilisation. However, difiiculty has arisen in attempting to construct pickup tubes of the electron image type which are adapted to operate with cathodepotential stabilisation because in such tubes it is desirable, for incidental reasons, for the scanning beam of electrons to approach the target at normal incidence, and it is also desirable for the axis of the electron optical system to be normal to the target so that the form of tube illustrated in the aforesaid Figure 1 is not applicable. One possible solution to the difiiculty which has been proposed is the use of a so-called double-sided target in a tube in which the electron gun and the photo-sensitive electrode are located at op- Claims. (Cl. 313-65) posite sides of the target so that the electron gun,

the target and the photo-sensitive electrode arev in line. However, double-sided targets have been found to possess a number of disadvantages.

The object of the present invention is to provide improved television or like transmitting apparatus embodying a pick-up tube of the electron image type, which does not involve the use of a double-sided target.

According to the present invention, there is provided television or like transmitting apparatus embodying a television transmission tube having a photoelectric cathode capable of converting a light image into an electron image, a target, and an electron gun mounted in a direct line between said cathode and said target for producing a beam of electrons for scanning said target, means for producing a scanning field between said gun and said target for scanning an electron beam generated by said gun over said target, and means for producing an oppositely directed scanning field between said cathode and gun so that an electron image can be projected from said cathode to said target without substantial deflection at the target.

In order that the said invention may be clearly understood and readily carried into eifect, the same will now be more fully described with reference to the accompanying drawing in which:

Figure 1 illustrates one example of a television transmitting apparatus in accordance with the present invention, and

Figure 2 illustrates a modification of the apparatus illustrated in Figure 1.

Referring to the drawing, the tube illustrated in Figure 1 comprises an evacuated glass envelope I having a window 2 at one end, on which is deposited a photo-electric cathode 3. At the other end of the tube I, there is provided a target comprising a thin sheet of dielectric material 4 facing the photo-sensitive electrode 3 and having a conductive signal plate 5 applied to the side of the sheet 4 opposite said electrode 3. An electron gun 6 is located between the electrode 3 and the sheet 4, the gun comprising a thermionic cathode 1, modulating electrode 8 and anode 9 which as shown are coaxial and have their common axes disposed transversely of the axis of the envelope l. The gun is also shown centrally disposed between the electrode 3 and the sheet 4, but the central disposition though desirable is not necessary. The electrode 3, the sheet 4 and the gun 6 are arranged in line, and the electrode 3 and sheet 4 are normal to the axis of the envelope I. The anode of the gun extends almost wholly across the envelope l and the diameter-of the gun 6 is arranged to be as small as possible to minimise the shadow cast on the dielectric sheet 4, the gun diameter being relatively enlarged on the drawings for convenience of illustration. Small circular apertures facing the sheet 4 are formed as shown on the modulator electrode 8 and anode 9 where the axis of the envelope l intersects these electrodes and electron emissive material is provided on the thermionic cathode l beneath these apertures. The cathode of the gun 6 is earthed, while the anode 9 is connected to a source IU of positive potential of, say, 300 volts and the modulator electrode is connected to a suitable source of bias potential H. A wall coating 12 of resistive material is provided on the inner surface of the envelope 1 between the gun 6 and the dielectric sheet 4, the coating being connected at the end nearer the gunto theafores'aid potential source it and at the other end to earth, so that there is a uniform fall in potential from left to right (in the drawing) along the wall coating 12, serving to produce efiectively planar equipotential surfaces between the gun 6 and the dielectric sheet 4 normal to the axis of the envelope 1. The equipotential surfaces decelerate the beam of electrons emanating from the electron gun so that the beam approaches the target with a low velocity. Between the gun 6 and the electrode 3, a similar resistive wall coating [3 is provided on the inner surface of the envelope likewise connected at the end nearer the gun to the potential source If! and at its other end to asource of negative potential of for example, 500 volts negative, the equipotential surfaces established by the wall coating l3 serving to accelerate elec tron images released from the electrode 3 towards the gun 6. The electrode 3 is connected to the source it, and an optical system indicated diagrammatically at I4 is provided for focussing a light image of the object for transmission on to the electrode 3.

The apparatus also comprises scanning coils indicated diagrammatically at It disposed between the gun 6 and the dielectric sheet 4 and supplied with sawtooth waveform currents of line and frame frequency for deflecting the electron beam from the gun 6 in mutually perpendicular directions, so as to cause the beam to scan the dielectric sheet 4 in known manner. The part of the envelope l to the right of the gun 6 is enclosed in a focussing solenoid ll coaxial with the envelope I and the signal plate 5 is connected to ground via a load impedance Hi from which a signal output can be obtained by means of a connection l9. Between the gun 6 and the electrode 3, further scanning coils are provided identical with the coils it but connected in reverse sense and the coils 2B are arranged to be supplied with sawtooth waveform currents identical with those applied to the coils It. The part of the tube to the left of the gun 6 is enclosed in a focussing solenoid 2 l as shown. Therefore when a light image is focussed on the electrode 3, the electron image released therefrom is accelerated by the wall coating l3 and projected past the sun 6 on to the dielectric sheet A at substantially normal incidence with a velocity corresponding to about 500 electron volts, the electron image being focussed by the solenoids 2| and [1. It therefore causes copious secondary-electron emission from the dielectric sheet 4 and produces in known manner a charge image on the target corresponding to the light image, by positively charging the elementary capacities of the target. The deflections imparted to the electron image by the coils 20, being of opposite sense to those imparted by the coils IS, serve effectively to cancel the deflections produced by the latter coils so that the electron image is projected on the dielectric sheet t effectively without resultant deflection. Two positions of extreme deflection in the vertical sense of the electron image are indicated in Figure 1 by dotted lines and dot-and-dash lines respectively. However, since said electron beam from the gun 6 only traverses the deflecting fields produced by the coils 16, said beam scans the dielectric sheet 4 and periodically stabilises it at cathode potential, in the normal manner, setting up signals representative of said charge image across the impedance [8. The solenoid I! focusses the scanning beam. By'maintaining the part of the wall coatings l2 and l3'near the gun and the anode 9 of the gun 6 at the same potential, distortion of the electron image due to pulling of electrons by the anode 9 is substantially avoided. Nevertheless, some electrons from the electron image are intercepted by the anode 9 so that in effect a shadow of the gun scans the di electric sheet 4 and it is therefore desirable that the gun should be symmetrical with respect to the axis of the envelope I. Moreover, with the arrangement shown in Figure 1, it is necessary for the diameter of the envelope to be at least twice the length of the diagonal of the area of the electrode 3 on which the light image of the object for transmission is projected.

In order to avoid the risk of loss of definition of the electron image due to inequalities in the deflections imparted on opposite sides of the gun 6, it may be desirable to effect the scanning of the electron image in an electric-field-free space. This may be achieved as illustrated in Figure 2 by replacing the resistive wall coatings l2 and I3 by wall coatings 22 and 23 of effectively zero resistance, terminated as shown in Figure 2 some distance from the electrode 3 and the dielectric sheet 4 and by providing high transparency mesh electrodes 24 and 25 at the outer ends of said wall coatings 22 and 23, the meshes being disposed normal to the axis of the envelope l, and connected to said coatings 22 and 23. The tube construction illustrated in Figure 2 is otherwise similar to that shown in Figure 1 and corresponding parts bear the same reference numerals.

In order to reduce the amplitude of the scan-. ing deflections of the electron image, a further focussing solenoid may be provided coaxial with the envelope and extending over the region of the gun 6, said further solenoid being arranged to produce an axial magnetic field-twice as strong as that produced by the solenoids l1 and 21 at the dielectric sheet 4 and at the electrode 3. This would also enable the diameter of the tub envelope l to be reduced.

While the invention has been described as applied to apparatus employing pick-up tubes of the kind which operate with cathode potential stabilisation, the invention may also b applicable to apparatus embodying other types of pick-up tubes.

What I claim is:

1. In apparatus for generating electrical signals representative of a light image, a pick-up tube comprising a photoelectric cathode for converting a light image to an electron image, a

target positioned for exposure to an electron image from said cathode, an electron beam source mounted in a direct line between said cathode and said target, deflecting means for setting up an electron-deflecting field between said source and said target for scanning an electron beam from said source over said target, and further deflecting means for setting up an oppositely directed electron-deflecting field between said cathode and said source proportioned to balance out deflections imparted by said first deflecting means to an electron image projected from said cathode to said target, whereby an electron image so projected is undefiected at said target.

2. In apparatus for generating electrical signals representative of a light image, a pick-up tube comprising a planar photoelectric cathode surface for converting a light image to an electron image, a planar target surface facing said cathode surface and parallel thereto, an electron gun mounted'in a direct line between said cathode surface and-said target surface for generating an electron beam directed at 'said'target surface, means for setting up a longitudinal magnetic field normal to said surfaces for focussing an electron image from said cathode surface and an electron beam from said gun normally onto said target surface, deflecting means for setting up an electron-deflecting field between said gun and said target surface for scanning an electron beam from saidgun over said target surface, and further deflecting means for setting up an equal and opposite electron-deflecting field between said cathode surface and said gun to balance out deflection imparted by said first deflecting means to an electron image projected from said cathode surface to said target surface, whereby an electron image so projected is focussed without deflection at said target surface.

3. In apparatus for generating electrical signals representative of a light image, a pick-up tube according to claim 2, said electron gun comprising at least a cathode and an anode of tubular form surrounding said cathode with its axis parallel to said photoelectric cathode surface and said target surface, said anode having an aperture to allow emergence of an electron beam directed at said target surface.

4. In apparatus for generating electrical signals representative of a light image, a pick-up tube comprising a planar photoelectric cathode surface for converting a light image to an electron image, an electrostatic charge storage target having a planar electron-image-receiving surface facing said photocathode surface and parallel thereto, an electron gun including at least a cathode and an anode mounted in a direct line between said photocathode surface and said target surface, means for setting up a longitudinal magnetic field normal to said surfaces for focussing an electron image from said photocathode surface and an electron beam from said gun normally onto said target surface, polarizing means for polarizing the electrodes of said tube to cause an electron image from said photocathode surface to be projected with high velocity on said target surface to form a positive charge image thereon by releasing secondary electrons and to cause an electron beam from said gun to approach said target surface with a low velocity, deflecting means for setting up an electron-deflecting field between said gun and said target surface for scanning an electron beam from said gun over said target surface to periodically stabilize said surface approximately at the potential of said gun cathode, and further deflecting-means for setting up an oppositely directed electron-defleeting field between said photocathode surface and said gun proportioned to balance out deflection imparted by said first deflecting means to an electron image projected from said cathode surface to said target surface.

5. In apparatus for generating electrical signals representative of a light image, a pick-up.

tube comprising a planar photoelectric cathode surface for converting a light image to an electron image, a planar target surface facing said cathode surface and parallel thereto, an electron gun including electrodes polarized with different potentials and mounted in a direct line between said cathode surface and said target surface for generating an electron beam directed at said target surface, means for setting up a longitudinal magnetic field normal to said surfaces for focussing an electron image from said cathode surface and an electron beam from said gun normally onto said target surface, defleeting means for setting up an electron-deflecting field between said gun and said target surface for scanning an electron beam from said gun over said target surface, further deflecting means for setting up an equal and opposite electron-deflecting field between said cathode surface and said gun to balance out deflection imparted by said first deflecting means to an electron image projected from said cathode surface to said target surface, a screening electrode surrounding the electron path between said cathode surface and said gun, a further screening electrode surrounding the electron path between said gun and said target surface, and polarizing means for maintaining the ends of said screening electrodes nearer said gun at the same potential as the most positively polarized electrode of said gun.

6. Inapparatus for generating electrical signals representative of a light image, a pick-up tube according to claim 5, said screening electrodes being of low resistance so that a uniform potential is maintained by said polarizing means between the ends of each of said electrodes.

7. In apparatus for generating electrical signals representive of a light image, a pick-up tube according to claim 5, said screening electrodes being of substantial resistance, and said polarizing means being arranged to maintain the ends of said screening electrodes nearer said gun at approximately equal potentials and the other ends of said electrodes at more negative potentials, with the latter end of the first-mentioned screening electrode most negative.

8. In apparatus for generating electrical signals representative of a light image, a planar photoelectric cathode surface for converting a light image to an electron image, an elec trostatic charge storage target including a planar electron image receiving surface facing said photocathode surface and parallel thereto and a conductive signal plate in capacitative association with said image-receiving surface, a load impedance connected to said signal plate, an electron gun including at least a cathode and an anode mounted in a direct line between said surfaces for generating an electron beam directed at said image-receiving surface, means for setting up a longitudinal magnetic field normal to said surfaces for focussing an electron image from said photocathode surface and an electron beam from said gun normally onto said image receiving surface, deflecting means for setting up an electron-deflecting field between said gun and said image-receiving surface for scanning an electron beam from said gun over said image-receiving surface, further deflecting means for setting up an equal and opposite electron-deflecting field between said photocathode surface and said gun to balance out deflection imparted by said first deflecting means to an electron image projected from said cathode surface to said image-receiving surface, polarizing means for polarizing said photocathode surface, said gun cathode and said gun anode with successively more positive potentials and for polarizing said signal plate via said load impedance with substantially the same potential as that of said gun cathode, said polarizing potentials being proportioned to cause an electron image from said photocathode surface to be projected with a high velocity on said photocathode surface to form a positive charge image thereon by releasing secondary electrons and to cause an electron beam from said gun to approach said image-receiving surface with a low velocity to periodically stabilize said image-receiving surface approximately at the potential of said gun cathode.

9. In apparatus for generating electrical signals representative of a light image, a pick-up tube according to claim 8 further comprising a screening electrode surrounding the electron path between said photocathode surface and said gun, and a further screening electrode surrounding the electron path between said gun and said image-receiving surface, and means for pclarizing the ends of said screening electrodes nearer said gun at least as positively as said gun anode.

10. In apparatus for generating electrical signals representative of a light image, a pick-up tube comprising a planar photoelectric cathode surface, a planar target surface facing said cathode surface and parallel thereto, an electron gun mounted in a direct line between said cathode surface and said target surface, a focussing solenoid with its axis normal to said planar surfaces disposed on the photocathode side of said gun, a second focussing solenoid coaxial with said first solenoid disposed on the target side of said gun, and a third focussing solenoid surrounding said gun for increasing the focussing field for the region of said gun, said solenoids being dimensioned to focus an electron image from said cathode surface and an electron beam from said gun normally on said target surface, deflecting means for setting up an electron-deflecting field between said gun and said target surface for scanning an electron beam from said gun over said target surface, and a further deflecting means for setting up an equal and opposite electron-deflecting field between said cathode surface and said gun to balance out deflection imparted by said first deflecting means to an electron image projected from said cathode surface to said target surface, whereby an electron image so projected is focussed without deflection at said target surface.

REGINALD SIDNEY WEBLEY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,189,988 Lester Feb. 13, 1940 2,203,734 Lubszynski June 11, 1940 2,251,992 Flory et al. Aug. 12, 1941 2,423,124 Teal July 1, 1947 

